The present invention relates generally to a duplexer for an ultrasound imaging system, and in particular to a duplexer for selectively connecting an ultrasound transducer element to either a transmitter or a receiver. A duplexer of this general type is disclosed, for example, in German publication DE-A-33 01 967.
The present invention is related to an invention described in an application entitled "Duplexer Including a Field-Effect Transistor for Use in an Ultrasound Imaging System," filed concurrently with the present application by the same inventors. The disclosure of the copending application is explicitly incorporated herein by reference.
Ultrasound imaging systems generally operate according to a "pulse-echo method." Such systems are capable of alternately functioning in two basic modes: transmit and receive. In the transmit mode, one or more piezoelectric transducer elements, preferably arranged in a linear or two-dimensional array, are excited to high-frequency oscillation by electrical pulses emitted by a transmitter, thereby generating an ultrasound pulse that may be directed at an object to be imaged. This ultrasound pulse is echoed back towards the transducer from some point within the object; for example, at boundary layers between two media with differing acoustic impedance. Then, in the receive mode, the "echo pulse" is received by the transducer element and converted into a corresponding electrical input signal (i.e., the "echo signal") that is fed to a receiver equipped with sensitive preamplifiers for enhancing the signal. The amplified signal may then be fed to a signal processor for evaluating the echoed image data to generate a visual image.
The transmit pulse and the echo pulse must be separated from one another for the two operating modes of the ultrasound imaging system to operate effectively. To this end, a transmit/receive filter, known as a duplexer, is associated with each transducer element in the ultrasound imaging system. Each duplexer selectively connects a respective transducer element to either the transmitter or the receiver, depending on whether the transducer element is operating in the transmit mode or the receive mode.
Decoupling the transmitter from the receiver is principally desirable because of the differing amplitudes of the transmitted and received signals. For instance, the transmit pulses used to excite the transducer elements have comparably high voltage amplitudes, typically greater than 100 V. On the other hand, the received echo signals are comparatively weak, requiring amplification by sensitive preamplifiers in the receiver. The duplexer must therefore electrically decouple the receiver from the transmitter during the transmit mode to protect the preamplifiers from being damaged by the strong transmit pulses coming from the transmitter. Decoupling the receiver from the transmitter is also desirable during the receive mode to prevent noise generated by the transmitter from disrupting the signals input to the receiver. Similarly, decoupling the transmitter from the receiver is desirable during the transmit mode to prevent the receiver from reacting to the transmitter in a manner that might disrupt or distort the shape of the transmit pulse.
In addition to these fundamental operational requirements for a duplexer in an ultrasound imaging system, further conditions must often be met in practice. For example, small transducer elements, such as those typically found in linear or two-dimensional arrays, generally have a high internal impedance. The ohmic and, above all, the capacitive loading caused by the duplexer should therefore be kept as small as possible. Likewise, the internal current consumption of the duplexer should be minimized to limit power loss in the array. Finally, duplexers should be designed as small as possible to allow them to be easily integrated with the array.
Duplexers for ultrasound imaging systems can be designed to be active (i.e., controlled by a switching signal) or passive (i.e., functioning automatically).
Duplexers generally consist of two subcircuits. The first subcircuit couples the transducer element to the transmitter in the transmit mode, and decouples the transducer element from the transmitter in the receive mode. In addition, this first subcircuit decouples the transmitter from the receiver in the receive mode. Conversely, the second subcircuit couples the transducer element to the receiver in the receive mode, and decouples the transducer element from the receiver in the transmit mode. In addition, this second subcircuit decouples the receiver from the transmitter in the transmit mode. The two subcircuits and the transducer element are electrically connected to one another at a common connection point.
An inverse-parallel circuit consisting of two p-n diodes arranged with opposing polarity and connected respectively between the transducer element and the transmitter can serve as a suitable first subcircuit of a duplexer. In the transmit mode, only the pulse height of the transmit pulses is then reduced by the comparably low conducting-state voltage of the diodes (typically 0.7 V). In the receive mode, the two diodes represent only a small capacitive load, since the voltages occurring in the received echo signals are generally considerably smaller than the conducting-state voltage of the diodes. A circuit of this type is disclosed in U.S. Pat. No. 5,271,403.
A duplexer for an ultrasound imaging system having a linear ultrasound array of transducer elements and two specially-designed subcircuits having the above-described functions is disclosed in German publication DE-A-33 01 967. In that device, the first subcircuit (i.e., the "transmitter switch") consists of a p-n diode as a first switch, which is connected between the transducer element and the transmitter, and a second switch, which is connected in parallel to the transducer element between a connection point (situated between the first switch and the transducer element) and ground to provide better reverse attenuation. In the second subcircuit (i.e., the "receiver switch"), a circuit consisting of a resistor, a bidirectional transient absorption Zener diode for protecting the receiver input, and a receiver amplifier is connected between a switch manufactured using CMOS technology (which is controllable with a TTL logic level and exhibits high reverse attenuation) and the transducer element. The receiver switch is connected to the receiver input.
A significant failing of known duplexers, such as the ones just described, is the existence of relatively high leakage capacitances toward ground and toward the control lines. Moreover, increased loading of the transducer elements, exhibited as signal sources, is often associated with these leakage capacitances.